Electrochemical production of 1,4-dihydro aromatic compounds

ABSTRACT

AROMATIC HYDROCARBON COMPOUNDS ARE SELECTIVELY REDUCED ELECTROCHEMICALLY TO THEIR 1,4-DIHYDRO DERIVATIVES IN AN AMMONIATED SOLVENT-ELECTROLYTE SYSTEM UNDER CONDITIONS SELECTED TO EFFECT OXIDATION OF AMMONIA.

United States Patent O 3,699,020 ELECTROCHEMICAL PRODUCTION OF1,4-DIHYDRO AROMATIC COMPOUNDS John F. Connolly, Chicago, and Robert J.Flannery, Olympia Fields, 111., and John D. McCollum, Munster, 1nd,,assignors to Standard Oil Company, Chicago, Ill. No Drawing. Filed Mar.27, 1970, Ser. No. 23,466 Int. Cl. C07b 1/00, 29/06 US. Cl. 204-73 26Claims ABSTRACT OF THE DISCLOSURE Aromatic hydrocarbon compounds areselectively reduced electrochemically to their 1,4-dihydro derivativesin an ammoniated solvent-electrolyte system under conditions selected toeffect oxidation of ammonia.

BACKGROUND OF THE INVENTION Partial reduction of aromatic compounds totheir respective 1,4-dihydro derivatives has been both difficult andexpensive. Ditficulty attaches both to the selective formation ofdihydro derivatives and to the preparation of the 1,4-dihydro isomer tothe substantial exclusion of the 1,2-isomer. Methods known in the artgenerally consume expensive reagents so that the cost of the desiredproduct becomes prohibitively great.

Methods for reduction of an aromatic compound (A) to its 1,4-dihydroderivative (AH can be represented stoichiometrically by the followingequations:

Equation 1 represents direct catalytic hydrogenation, usually atelevated temperature and pressure. This method is impractical because oflow yield and difficulty in adequately purifying the desired product.

Equation 2 represents the Birch sodium-alcohol synthesis and ischaracterized by high chemicals consumption and cost. The analogouslithium-amine synthesis is likewise expensive and yields dihydroderivatives only when an alcohol is added to the system.

Equation 3 is illustrative of the electrochemical reduction systemrecently reported by Misono et al. for benzene hydrocarbons in J.Electrochem. Soc., 115, 266 (1968) and in US. Pat. No. 3,485,726. Thissystem was extended to naphthalene hydrocarbons as reported in Bull.Chem. Soc., Japan, 41, 2921 (1968). While selectivity of reduction tothe l,4-dihydro isomer is high, the process suffers limitations thatrender the cost too great for consideration when contemplatinglarge-scale operation. Large inventories are required for both thetetraalkyl ammonium bromide electrolyte and the diethylene glycoldimethyl ether (diglyme) solvent. The electrolyte is consumed in theprocess (at the rate of 6 pounds per pound of dihydro naphthaleneproduced) as bromine is liberated. The separator required to isolatethis bromine from the cathode has a high electrical resistance, greatlyincreasing power cost. Because the separator is not ideal, brominedifiusion must be reduced by keeping the concentration low. Thisrequires a large volume of solvent in the anode chamber. Other solventssuch as acetonitrile may be used but these are also expensive.

Electrochemical reduction of benzene and its derivatives employing anamine solvent and lithium halide electrolyte has been reported byBenkeser et al. in J. Am. Chem. Soc., 86, 5272 (1964). An undivided cellwas required for preparation of 1,4-dihydro isomers. In a divided celllithium amide was formed in the reaction and catalyzed isomerization tothe conjugated 1,2-dihydro form Patented Oct. 17, 1972 which was readilyreduced further. Solvent was oxidized to condensation products.

More recently, electrochemical reduction of benzene to1,4-dihydrobenzene has been effected employing an alkali halide and analcohol in liquid ammonia (US. Pat. No. 3,493,477) or a lithium halideand an alcohol in hexamethyl phosphoramide (US. Pat. No. 3,492,207). Theformer requires rigorous exclusion of iron and water, operates at verylow temperature or high pressure and provides a low current efiiciency.The latter operates with a cell divider and exhibits a satisfactorycurrent efficiency but at low conversion levels.

SUMMARY OF THE INVENTION This invention relates to an improved processfor electrochemical reduction of aromatic compounds selectively to thecorresponding 1,4-dihydro derivatives.

Specifically, this invention relates to an electrochemical reductionprocess for aromatic compounds employing ammonia as the anodedepolarizer, under conditions selected to yield dihydro aromaticderivatives, as the 1,4- or non-conjugated isomer, at the cathode whileliberating DESCRIPTION OF THE INVENTION Our invention effects thepartial, selective reduction of aromatic compounds to 1,4-dihydroaromatic compounds in an electro-chemical cell containing the aromatichydrocarbon dissolved in a solvent-electrolyte and operating at acurrent density (and cathode potential) insufficient to effect evolutionof much hydrogen. Ammonia is added to the solution in an amount at leastequal to and preferably in excess of that stiochiometrically requiredfor service as a proton donor (or replenisher), whereby the ammonia isoxidized electrochemically and molecular nitrogen is formed. Thesolvent-electrolyte system comprises an aprotic solvent together withminor quantities of water and a salt soluble therein, preferably aquaternary ammonium halide. Electrolysis is continued untilsubstantially all of the aromatic compound has been reduced. Afterseparation from thesolvent components the dihydro aromatic product isseparated by conventional means, such as distillation, from minoramounts of aromatic compound and more highly reduced derivatives. The1,4-dihydro aromatic can be purified by distillation because of theabsence of the close-boiling 1,2-dihydro isomer. Very little electrolyteis consumed. Accordingly, chemicals consumption is limited tosubstantially a stoichiometric quantity of ammonia which is bothinexepnsive and readily available in pure form.

In our electrochemical process the reactions can he represented as:

Operating and economic advantages for this process include the need foronly a small inventory of quaternary ammonium salt and of solvent. Thisgreatly lowers chemicals cost and plant size. No separator is requiredin the electrolytic cell and this greatly reduces electric powerconsumption while simplifying plant construction.

Suitable aromatic compounds for the practice of this invention includearomatic hydrocarbons, their alkyl and cycloalkyl derivatives, andsubstituted aromatic hydrocarbons having polar substituents such ascarboxyl, cyano, halo, and like groups. Examples of such compoundsinclude alpha-methylnaphthalene, beta-methyl-naphthalene, binaphthyl,benzoic acid, phthalic acid, terephthalic acid, isophthalic acid andnaphthoic acids. Especially suitable compounds include naphthalene andbenzo-substituted naphthalenes such as triphenylene(1,'2,3,4-dibenzonaphthalene).

Aromatic hydrocarbons such as naphthalene and the methylnaphthalenes arereadily obtained in substantially pure form by distillation frompetroleum reformer bottoms (by-product from aromatic gasolinemanufacturing processes such as Ultraforming) or from coke-ovendistillate.

Suitable aprotic solvents include acetonitrile, dimethylformamide,dimethyl sulfoxide, alkyl ethers, glycol ethers, and especially alkyleneglycol alkyl ethers. These are characterized by their lack of protondonor capacity. We particularly prefer to employ diethylene glycoldimethyl ether, generally known as diglyme, because of its solvent powerfor both aromatic hydrocarbons and polar compounds.

Our electrochemical process requires the presence of a small quantity ofwater (see the equations above), which is continuously regenerated, andalso an electrolyte salt which must be soluble in the combination ofwater and aprotic solvent. For this purpose we prefer to use tetraalkylammonium halides. Both chlorides and bromides are satisfactory. Thealkyl substituent is preferably a butyl group. With propyl groupssolubility of the salt is borderline while pentyl groups may notbe'sufiiciently stable. We particularly prefer to use tetrabutylammonium bromide.

When preparing the electrolysis solution the concentrations of thevarious components can be varied within rather wide limits. An effectiveelectrolysis medium will be obtained when the respective components ofthe solution are employed in the following concentration ranges:

Wt. percent Diglyme 50-80 Water 3-10 Tetrabutyl ammonium bromide 2-25Naphthalene -30 Care should be taken to maintain a one-phase liquidmedium so that, for example, the maxima indicated above for water, salt,and naphthalene will not be present simultaneously. When preparing1,4-dihydronaphthalene, we prefer to use solutions containing to 25 wt.percent naphthalene, 5 to 10 wt. percent tetrabutyl ammonium bromidesalt and 3 to 5 wt. percent water together with diglyme.

Ammonia is consumed in our process and accordingly should always bepresent in some excess concentration over that stoichiometricallyrequired for the electro-reduction. A high concentration of ammonia neednot be employed, but it has been found convenient to maintain thesolvent-electrolyte system about 1 molar in ammonia. This can bemaintained approximately by continuously bubbling ammonia into thesolution contained in the electrolysis cell during reduction. If thesolution is continuouly removed from the cell for cycling through acooling system, ammonia can be conveniently injected into the line,preferably downstream of the pumping means.

In the practice of our invention carbon anodes have been satisfactory.The carbon may be impregnated with a metal; e.g. platinum, althoughmetals tend to oxidize and then plate out on the cathode. The cathodemust be a material exhibiting a high hydrogen overvoltage such as lead,mercury, aluminum, tin, zinc and cadmium. Lead and mercury areespecially suitable.

In our preferred system ammonia is the most easily oxidized entity sothat oxidation of bromide ion is essentially eliminated. The operatingvoltage should avoid electrolysis of water and consequent liberation ofhydrogen. In general, an impressed potential in the range from about 5to about 25 volts, and preferably from 6 to 15 volts, may be employed.Current density will accordingly vary within the range from 20 to- 50,and desirably 20 to 35, amperes per square foot.

Electrolysis temperature is not a critical variable and reductions mayreadily be conducted within the range from 70 to 140 F. Thesolvent-electrolyte solution must be fluid at the selected temperatureand the solubility of ammonia must be adequate to satisfy stoichiometricrequirements. Operation at or near room temperature to F.) is preferred.During electrolysis part of the solution may be continuously removedfrom the cell, passed through a cooling coil, injected with ammonia, andreturned to the cell. i

As our selective electro-reduction progresses, the impressed voltage isincreased to maintain the desired current density whenever there is aslow erosion of the carbon anode (for example, a loss amounting to about0.001 inch per 24 hours). Salt consumption is low, although it tends toincrease at low water concentrations while hydrogen evolution occurs athigh water concentrations. No separator is required in our system.Indeed, presence of a separator to afford compartments and minimizemixing between reagents could lead to an inoperable system by effectingprecipitation of, for example, ammonium bromide on the separator frit,and thus gradually shut off the electrochemical reaction.

When the reduction operation is complete the cell contents are usuallytransferred to a still and heated under mild vacuum to remove solvent,water and ammonia. The still bottom is extracted with a light aliphaticsolvent (e.g.. hexane). Tetraalkyl ammonium halide is recovered from theresidue and a crude hydro-aromatic product is recovered from theextract. If susceptible to distillation this crude product is readilyresolved into fractions containing tetra-hydro, di-hydro and unreactedaromatic compounds. The di-hydro fraction is free of the 1,2-dihydroisomer so that conventional distillation or extractive distillation canafford the desired 1,4-dihydro isomer in high purity.

In continuous operation, our process comprises the use of banks ofelectrolytic cells, each bank including a plurality of cells arranged ina line and having abutting walls to conserve space. Each cell isrelatively narrow and deep and fitted with thin, flat facing electrodesdisposed vertically. In one such arrangement each anode (carbon) andeach cathode (any conductor having a high hydrogen overvoltage andpreferably lead) is punched in the center and supported from a hollowrod extending upwardly. Either or both rods may be adapted to adjusthorizontally the spacing between the electrodes to any desired distance.Inlet and exit lines are provided for flow of solution through the cell.Ammonia is added continuously to each cell through the hollow supportrods or alternatively is injected into the inlet lines.

Within a cell-bank the electrical connections to the electrodes arearranged in series. Between cell-banks electrical connections may beeither in series or in parallel arrangement. The solution flow patternmay be arranged in parallel within a cell-bank and in series between aplurality of banks as required to give substantially complete reductionto the 1,4-dihydro stage. Provision can be made for cooling the solutionoutside the cells as needed. Solution may also be recycled, in whole orin part, as required to effect a conversion of at least 80%. Whenrecovering product, as by distillation or extractive distillation, partof the 1,4-dihydro product may be removed in a bottom stream togetherwith the less volatile feed material and recycled therewith toanappropriate cell-bank.

The 1,4-dihydro aromatic compounds are highly desirable because they arecapable of undergoing the known reactions of olefinic systems inasmuchas they are nonconjugated olefins. They are capable of undergoingpolymerization, formation of glycols and halohydrins and oxidation topolybasic acids. One reported use for 1,4-dihydro naphthalene (Chem.Eng. News 47, No. 45, 46 [1969]) is in modifying diene polymerizationsto afiord special properties in synthetic rubbers.

EMBODIMENTS OF THE INVENTION The following embodiments are presented asillustrative of our invention and are not to be construed as limitationsupon its reasonable scope.

EXAMPLE I An electrolytic cell was fitted with a fiat, circular carbonanode (1.5-inch diameter, A; inch thick) disposed horizontally andsupported above a pool of mercury in the bottom of the cell, serving asa cathode. The carbon anode was platinized to the extent of 20 mg./cm.surface. A sparger tube was inserted in the cell and a coated stirringbar was floated on the mercury. Lead wires connected the electrodes to apower source. The cell was filled with 10 grams of a solution containing20 wt. percent naphthalene, 7 wt. percent tetrabutyl ammonium bromide, 7wt. percent water and the remainder diethylene glycol dimethyl ether(diglyme). A potential of 23 volts was impressed across the electrodeswhile ammonia was bubbled into the cell through the sparger tube. Thecurrent density was 40 amperes/square foot. After the reaction wasstopped, crude product was recovered by distillation to remove solventand extraction with n-hexane to remove electrolyte. The crude productwas analyzed and contained 1 wt. percent naphthalene 6, wt. percentTetralin, and 93 wt. percent 1,4-dihydronaphthalene. Current efiiciencywas 70% EXAMPLE II Example I is modified employing tetrabutyl ammoniumchloride as the electrolyte salt. Selectivity is unchanged and currentefiiciency is somewhat less.

EXAMPLE HI Two percent triphenylene was dissolved in a solvent (90 partsdiglyme and 10 parts water) containing 25 wt. percent tetra-n-butylammonium bromide. Electrolysis was conducted using a carbon anode andmercury cathode at a current density of 30 amperes/square foot. The soleprodnot was 1,4-dihydro triphenylene (1,4-dihydro5,-6,7,8-dibenzonaphthalene EXAMPLE IV Two percent triphenylene isdissolved in a solvent (95 parts diglyme and parts water) containing 25wt. percent tetra-n-butyl ammonium bromide. Ammonia is bubbled incontinuously. Electrolysis is conducted as in Example III and the sameproduct is obtained.

EXAMPLE V An electrolytic cell was fitted with disc electrodes 9 inchesin diameter and inch thick. A carbon anode was disposed horizontallynear the bottom of the cell. A lead cathode was punched out at itscenter to receive a hollow cylindrical suport rod and was maintainedparallel to and above the anode by insulating spacers placedtherebetween. The separation between the electrodes was 0.03 inch. Asuction tube was extended from the cell to a circulating pump. Theefiluent line from the pump then was passed through a cooling bath andadapted to returning to the cell through the hollow support rod. Ammoniawas injected continuously through a T in the etliuent line downstreamfrom the cooling coil. Sheathed lead wires connected the electrodes to apower source. A voltmeter and an ammeter were provided in the circuit.The cell was filled wtih a solution containing 20 wt. percentnaphthalene, 4 wt. percent water, 7 wt. percent tetrabutyl ammoniumbromide and 69 wt. percent diglyme. Ammonia was then added to provide anapproximately 1 molar concentration. While circulating the solution andcontinuously injecting ammonia, a potential of 8 volts was impressedacross the electrodes. The current density was 33 amperes/ square foot.Crude product was recovered and distilled to yield 3 wt. percentnaphthalene, 7 wt. percent Tetralin and wt. percent1,4-dihydronaphthalene. Current efficiency was 60% EXAMPLE VI Theapparatus of Example V was employed in a 455- hour run, conducted inseven batch operations of 65 hours each. Electrodes were not cleaned ormoved between batches. Each batch employed 8 kg. solution containing 20wt. percent naphthalene, 4 wt. percent water, 7 wt. percent tetrabutylammonium bromide, 69 wt. percent diglyme and ammonia to provide 1 molarconcentration. Solution was pumped (with ammonia injection) to maintaina temperature of F. and to provide a velocity of 3 feet/second in thespace between the electrodes. A potential of 8 volts was used at thestart of the run to provide a current density of 33 amperes/square foot.During the final batch operation the potential had been increased to 14volts. After 455 hours operation the spacing between electrodes hadincreased to 0.06 inch and there was a coating on the cathode. Currentefficiency was nearly constant, being 63% for the first batch and 65%for the last batch. Crude product from the combined batches wasrecovered by distillation and hexane extraction of the still bottoms.The crude material (9.55 kg.) contained 3 wt. percent unreactednaphthalene, 7 wt. percent Tetralin and 90 wt. percent 1,4-dihydronaphthalene.

EXAMPLE VII Example VI is modified by increasing the water content ofthe solution to 4.5 wt. percent. The initial potential is 6 volts toprovide a current density of 20 amperes/ square foot. In the final batchthe potential has increased to 8 volts. The yield selectivity (to1,4-dihydro naphthalene) is 93% and the current efiiciency is 65 Weclaim:

1. A process for the production of a 1,4-dihydro aromatic hydrocarboncompound comprising electrochemically reducing, at a cathode, anaromatic hydrocarbon compound contained in an organicsolvent-electrolyte system, introducing substantially anhydrous ammoniainto the solution and concurrently oxidizing said ammonia at an anode.

2. The process of claim 1 wherein the aromatic hydrocarbon compound is acondensed-ring aromatic hydrocarbon selected from the class consistingof naphthalene, alkyl-substituted naphthalenes, and benzo-substitutednaphthalenes.

3. The process of claim 1 wherein the organic solventelectrolyte systemcomprises a major amount of an aprotic organic liquid together withabout 3 to 10 wt. percent water and about 2 to 25 wt. percent of atetraalkyl-substituted ammonium halide, wherein the alkyl substituentscontain at least four carbon atoms and the halide is bromide orchloride.

4. The process of claim 3 wherein the aprotic organic liquid is adialkylene glycol dialkyl ether.

5. The process of claim 1 wherein ammonia is maintained in solution inthe organic solvent-electrolyte system in an amount in excess of thatrequired for the anodic oxidation.

6. A process for the electrochemical production of a 1,4-dihydroaromatic compound, comprising the steps of:

(a) dissolving an aromatic compound in an organic solvent-electrolytesystem comprising a dialkylene glycol dialkyl ether together with 3 to10 wt. percent of water and 2 to 25 wt. percent of a tetrabutyl ammoniumhalide, selected from the class consisting of chloride and bromide;

(b) introducing the solution into an electro-chemical cell having ametallic cathode and a carbon anode spaced therefrom;

(c) introducing substantially anhydrous ammonia into the solution;

(d) imposing a potential of at least volts between the anode and thecathode;

(e) conducting electrolysis, with oxidation of ammonia and evolution ofnitrogen gas at the anode, until a major portion of the aromaticcompound has been reduced;

(f) withdrawing the solution from the cell; and

(g) separating the 1,4-dihydro aromatic compound from the othercomponents of the solution.

7. The process of claim 6 wherein the dialkylene glycol dialkyl ether isdiethylene glycol dimethyl ether.

8. The process of claim 6 wherein the ammonium halide is tetrabutylammonium bromide.

9. The process of claim 6 wherein the organic solventelectrolyte systemcontains 3 to wt. percent water and 2 to 25 wt. perecent tetrabutylammonium bromide dissolved in diethylene glycol dimethyl ether.

10. The process of claim 6 wherein solution contains 10 to 30 wt.percent aromatic compound and is maintained at a temperature in therange from 70 to 140 F.

11. The process of claim 6 wherein the cathode metal is mercury or lead.

12. The process of claim 6 wherein ammonia gas is continuously bubbledinto the solution by means of a sparger located in the cell.

13. The process of claim 6 wherein a portion of the solution is cycledcontinuously by pumping from the cell through an external pipe andammonia is introduced continuously into the solution prior to returningto the cell.

14. The process of claim 6 wherein the imposed potential is in the rangefrom 5 to 25 volts.

15. The process of claim 6 wherein the current density is in the rangefrom 20 to 50 amperes per square foot.

16. The process of claim 6 wherein the aromatic compound is napththaleneand the product is 1,4-dihydro naphthalene.

17. The process of claim 6 wherein the aromatic compound is triphenyleneand the product is 1,4-dihydro triphenylene.

18. A process for the electrochemical production of 1,4-dihydronaphthalene, substantially free of the 1,2- isomer, comprising the stepsof:

(a) dissolving naphthalene in an electrolyte-solvent system comprisingdiethylene glycol dimethyl ether together with minor amounts of waterand tetrabutyl ammonium bromide, to yield a solution comprising to 25wt. percent naphthalene, 3 to 5 wt. percent water, 5 to 10 wt. percenttetrabutyl ammonium bromide, and 77 to 60 wt. percent diethylene glycoldimethyl ether;

(b) introducing the solution into an electrolytic cell, fitted withfiat, horizontal carbon anode and a flat lead cathode, disposed about 1millimeter above the anode and supported on a hollow, cylindrical rod;

(0) adding ammonia to the solution, to maintain about 1 molarconcentration of ammonia, by continuously circulating the solution fromthe cell through an external ammonia injection system and returning theammoniated solution to the cell through the hollow cathode support rod;

(d) maintaining the cell at a temperature in the range from 80 to 120 F.while impressing a potential in the range from 6 to 15 volts across theelectrodes, to provide a current density in the range from to 35 amperesper square foot;

(e) continuing the electrolysis until at least of the naphthalene hasbeen converted; and

(f) separating the 1,4-dihydro naphthalene from the resultant solution.

19. A continuous process for the electrochemical production of1,4-dihydro naphthalenic hydrocarbons, sub stantially free of therespective -.l,2-isomers, comprising the steps of (a) dissolving anaphthalenic hydrocarbon feedstock in a solvent-electrolyte systemcomprising diethylene glycol dimethyl ether together with minor amountsof water and tetrabutyl ammonium bromide, to yield a solution comprising15 to 25 Wt. percent naphthalenic hydrocarbon, 3 to 5 wt. percent water,5 to 10 wt. percent tetrabutyl ammonium bromide, and 77 to 60 wt.percent diethylene glycol dimethyl ether;

(b) flowing the solution through electrolytic cells arranged in aplurality of cell-banks, each cell being fitted with a thin, flat,vertical carbon anode and a thin, fiat, vertical, electricallyconducting cathode, having a high hydrogen overvoltage, spaced from theanode;

(0) adding ammonia to the solution prior to flowing through the cells,to maintain about 1 molar concentration of ammonia;

(d) maintaining the cells at -a temperature in the range from 80 to LF.while impressing a potential in the range from 5 to 25 volts across thecell electrodes, to provide a current density in the range from 20 to5'0 amperes per square foot;

(e) conducting the electrolysis until at least 80% of the naphthalenichydrocarbon has been converted; and

(f) separating the 1,4-dihydro naphthalenic hydrocarbon from the productsolution.

20. The process of claim 19 wherein the naphthalenic hydrocarbon isnaphthalene and the product is 1,4-dihydr0 naphthalene.

21. The process of claim 19 wherein the electrolytic cells within acell-bank are connected electrically in series.

22'. The process of claim 19 wherein parallel solution flow ismaintained through each cell-bank and series solution flow is maintainedbetween adjacent cell-banks.

23. The process of claim 19 wherein the anode position within each cellis fixed and the cathode is adjustably positioned horizontally inrelation thereto.

24. The process of claim 19 wherein at least a portion of the effiuentsolution from the last cell-bank is continuously removed from theprocess system, the remaining solution being returned to the system forprocessing through at least one cell-bank.

25. The process of claim 19 wherein the 1,4-dihydro naphthalenichydrocarbon is purified by extractive distillation and a portion of the1,4-dihydro naphthalenic hydrocarbon is recovered in a bottom streamtogether with unconverted feedstock and recycled through at least onecell-bank.

26. The process of claim 19 wherein the cathode is lead.

References Cited U.S. Cl. X.R. 20459

